When it comes time to replace a hot water system, one of the first questions most homeowners ask is:
What size do I actually need?
It sounds like a simple question. Four people live in the house, so buy the system recommended for four people.
Unfortunately, hot water does not quite work like that.
Two four-person households can have completely different hot water requirements depending on when people shower, how many bathrooms are being used at once, whether there is a bath, how long showers run and even how the hot water system itself heats and recovers.
Choosing the right hot water system is therefore less about finding the biggest tank you can fit beside the house and more about understanding how your household actually uses hot water.
Bigger Isn’t Automatically Better
Running out of hot water is frustrating, so it is tempting to simply install a larger system.
But oversizing can create its own problems.
A larger storage system usually costs more to purchase and may use additional energy keeping a greater volume of water hot. On the other hand, a system that is too small can leave the last person in the shower wondering where all the hot water went.
The goal is to find the right balance between:
- household size
- peak hot water demand
- heating and recovery time
- energy source
- available tariffs
- available space
- running costs
- future household requirements
That is why simply replacing an old 250-litre system with another 250-litre system is not always the best answer either.
Your household may have changed considerably since the original system was installed.
Start With How Many People Live in the House
Household size is still a useful starting point.
For conventional electric storage systems, tanks commonly range from relatively small units through to 250, 315 and 400-litre systems designed for larger households.
But the number of people living in the home only tells us part of the story.
Consider two households with four people.
In the first home, two adults shower in the morning and two children shower at night.
In the second home, four adults all shower between 6:30 am and 7:30 am before leaving for work.
The total number of people is identical.
The peak demand is completely different.
The second household needs significantly more hot water available during a much shorter period.
Your Shower Can Change the Calculation
Showers are one of the biggest contributors to residential hot water use.
A household with efficient showerheads and relatively short showers can get considerably more usable shower time from a tank than a household where everyone enjoys long, high-flow showers.
Then there is the classic teenage shower.
If somebody disappears into the bathroom for 20 minutes every morning, the number printed on the hot water tank may not be the problem.
It may simply be trying to supply far more hot water than the average household calculation assumes.
Before increasing tank capacity, it is worth understanding where the water is actually going.
One Bathroom Versus Three Bathrooms
The number of bathrooms also matters.
A three-bedroom home with one bathroom generally has a different hot water profile from a large family home with an ensuite, main bathroom and guest bathroom.
The question is not necessarily how many bathrooms exist.
It is how many are likely to be used simultaneously.
This becomes particularly important with continuous-flow gas hot water systems.
Continuous-flow systems do not rely on a storage tank. Instead, they heat water as it passes through the system.
Their capacity is therefore commonly considered in terms of how much hot water they can deliver at a particular flow rate.
Running one shower is very different from running two showers while somebody uses hot water in the kitchen.
Correct sizing still matters; it is simply measured differently.
Electric Storage: Tank Size Is Only Half the Story
Electric storage is common throughout North Lakes, Mango Hill, Griffin, Kallangur, Narangba and the wider Moreton Bay area.
With these systems, homeowners often focus entirely on tank capacity.
But there is another important question:
When can the system actually reheat?
Some electric hot water systems operate on controlled or off-peak electricity arrangements.
This can reduce the cost of heating water, but the system may only receive power during certain periods.
That means the tank needs to store enough hot water to carry the household through until the next heating period.
A system with access to power for longer periods may be able to recover during the day, potentially changing the amount of stored water required.
Two identical tanks can therefore perform very differently depending on how they are configured.
Heat Pumps Need to Be Sized Differently Too
Heat pump hot water systems have become increasingly popular, particularly for households with rooftop solar.
They use electricity differently from a traditional resistive electric element and can substantially reduce the electrical energy required to heat water.
But installing a heat pump does not mean tank size suddenly stops mattering.
You still need to consider:
- household demand
- tank capacity
- recovery time
- when the unit is scheduled to operate
- available rooftop solar generation
- installation location
- airflow around the unit
- backup heating behaviour
For a household with solar, there can be a strong advantage in heating water during the middle of the day when the house is generating electricity.
In effect, the hot water tank can store some of that energy as heat for use later.
But the system still needs enough capacity to make it through the household’s peak demand periods.
What About Gas Continuous Flow?
Continuous-flow gas systems solve the storage problem differently.
Instead of heating a tank of water in advance, they heat water when you turn on the tap.
That means you are not choosing between a 160-litre and 315-litre tank.
The important figure becomes the unit’s ability to supply enough hot water for the home’s expected simultaneous demand.
A smaller household using one shower at a time may have relatively modest requirements.
A larger household wanting to operate multiple showers simultaneously needs considerably more capacity.
Gas supply and pipe sizing can also affect what can actually be installed, so replacing one continuous-flow unit with another should still involve proper assessment rather than simply choosing the largest model available.
Why Does My Existing System Suddenly Feel Too Small?
This is something we regularly see.
A hot water system may have worked perfectly for years and then gradually start struggling to keep up.
Sometimes the household has changed.
Maybe children have become teenagers.
Perhaps another person has moved into the property.
Maybe a bathroom renovation added a larger showerhead or a second shower.
But there can also be a fault.
Reduced hot water capacity can sometimes be caused by problems such as:
- a failed heating element
- thermostat problems
- incorrect heating schedules
- valve issues
- reduced system performance
- problems with solar or heat pump boosting
So if your household suddenly starts running out of hot water when it never used to, do not automatically assume you need a larger system.
The existing system may need to be inspected first.
Think About the Next Ten Years, Not Just Today
Hot water systems are long-term purchases.
That makes future household needs worth considering.
A couple living in a home today may be planning a family.
A family with teenagers may have children preparing to move out.
A homeowner may be planning an ensuite or bathroom renovation.
Solar panels or a home battery may also be on the horizon.
These changes can alter what makes sense.
There is no point installing a substantially oversized system for a future that may never arrive, but known changes to the property should form part of the decision.
The Best Hot Water System Isn’t Necessarily the Biggest
The right system is the one that reliably supplies the hot water your household needs without unnecessarily increasing installation or running costs.
That requires looking at the whole house rather than simply counting bedrooms.
At RJT Plumbing & Gas Solutions, we consider the household, bathrooms, existing system, energy source, usage patterns and property setup before recommending a replacement.
We work with electric storage, heat pump, gas, solar and continuous-flow hot water systems throughout North Lakes and the wider Moreton Bay region.
Not Sure What Hot Water System Suits Your Home?
RJT has developed an interactive Hot Water System Recommender that can help narrow down the options based on your household and priorities.
You can compare different system types and get a clearer idea of what may suit your property before speaking with us.
And if your existing system is leaking, running out of hot water or has stopped working altogether, we can inspect the system and determine whether it makes more sense to repair it or replace it.
Need help with hot water in North Lakes, Mango Hill, Griffin, Kallangur, Murrumba Downs, Narangba or surrounding Moreton Bay suburbs? Contact RJT Plumbing & Gas Solutions for practical advice based on your home.
Frequently Asked Questions
What size hot water system do I need?
The right hot water system size depends on more than just the number of people in the home. Household size, shower habits, the number of bathrooms, peak usage times, the type of hot water system and its recovery rate all affect the correct size.
Is a bigger hot water system always better?
No. An oversized hot water system can cost more to purchase and may use more energy than necessary, while an undersized system may regularly run out of hot water. The goal is to match the system to the household's actual usage.
How does household size affect hot water system sizing?
Household size is a useful starting point, but two homes with the same number of occupants can have very different hot water demand. The timing of showers, number of bathrooms in use and individual water habits can significantly change the required capacity.
Why does my hot water system suddenly seem too small?
A hot water system that previously worked well may begin running out because household demand has changed or because the system has developed a fault. Problems with heating elements, thermostats, valves, boosting or heating schedules can reduce available hot water.
Do heat pump hot water systems need to be sized differently?
Yes. Heat pump systems should be selected based on household demand, storage capacity, recovery time, operating schedule and available solar generation. The tank still needs to hold enough hot water to meet the household's peak usage.
How are continuous-flow gas hot water systems sized?
Continuous-flow gas systems are generally sized according to the amount of hot water they can deliver at one time rather than tank capacity. Homes using multiple showers or hot water outlets simultaneously may require a higher-capacity unit.
Should I replace my old hot water system with the same size?
Not necessarily. Household needs may have changed since the original system was installed. Before replacing it, it is worth considering current hot water demand, energy costs, available tariffs, solar, household size and future plans for the property.
